Continous cell cultures obtained by in vitro transformation of bone marrow cells with retroviruses containing ras or raf and myc were found to exhibit characteristics of both B cell lineage any myeloid cells. Analyses of cloned derivatives of these cultures showed that lines with phenotypes of mature B cells or mature macrophages could be isolated but that these lines were clonally related. These results indicate that some B cells and myeloid cells can develop from a common progenitor. The role of altered myc expression in plasmacytoma development was studied by inoculating pristane-primed BALB/c mice with a retrovirus expressing avian v-myc. Over 25% of infected mice developed plasmacytomas that lacked chromosomal translocations affecting the c-myc locus. Instead, the tumors contained and expressed high levels of v-myc. These results provide the most direct evidence to date for a central role for altered myc expression in plasmacytomagenesis. Mice transgenic for the membrane form of human IgM were studied for expression of murine and human IgM on B cells. By flow cytometry, B cells but not T cells or macrophages were shown to express the transgene. In addition, it was found that B cells expressed either human IgM or mouse IgM indicating that the membrane form of IgM can signal allelic exclusion.